Method for reprocessing waste acid resulting from TiO2 production

ABSTRACT

Process for the production of gypsum and also of an iron-oxide pigment from the waste acid that accumulates in the course of the production of titanium dioxide in accordance with the sulfate process, characterized in that in a first stage a partial neutralization of the waste acid is effected with a calcium compound subject to precipitation and optionally direct separation of gypsum, subsequently the remaining solution is neutralized further in a second stage subject to precipitation of a deposit containing Ti, Al, Cr, V and optionally Fe(III) and from the solution containing iron sulfate that is obtained after separation of the solids an iron-oxide pigment is produced in a third stage by addition of alkaline compounds and also of an oxidizing agent.

BACKGROUND OF THE INVENTION

The invention relates to a process for the production of high-gradegypsum and also of iron-oxide pigments from waste acid that accumulatesin the course of the production of titanium dioxide in accordance withthe sulfate process.

The utilisation or at least harmless elimination of waste acid isprescribed at sites in Europe and at most other sites for the productionof titanium dioxide, so that over the course of time various processeshave been developed with a view to utilisation: in printed publicationEP-A 577 272 it is disclosed that a usable gypsum can be obtained fromthe waste acid by partial neutralisation with calcium carbonate. Themetal-sulfate solution remaining after separation of this material, alsodesignated as “white gypsum”, is then brought to a pH value of about 9by addition of CaO or CaCO₃, the material obtained in the process, alsodesignated as “red gypsum”, having to be disposed of. In so doing, onthe one hand the opportunity for recycling of valuable raw materials islost, and on the other hand valuable landfill area is extensively usedup, since, depending on the titanium raw material, 1 to 2.5 t of thiswaste accumulates per tonne of TiO₂ pigment produced.

Another process, described in EP-A 0 133 505, for reprocessing the wasteacid avoids these disadvantages by practically the entire amount ofwaste acid accumulating being re-used for the production of TiO2, thewaste acid being firstly concentrated and, after separation of thefilter salts that are precipitated out in the process, the 65-% to 85-%sulfuric acid being employed again for the purpose of decomposing ore.SO₂ is obtained from the filter salts by thermal dissociation, and puresulfuric acid or oleum, which is likewise re-used for the purpose ofdecomposing ore, is obtained from the SO₂. Although this processminimises the consumption of raw materials, it is very energy-intensiveand therefore costly.

A further process for reprocessing waste acid, described in printedpublication U.S. Pat. No. 3,016,286, involves neutralisation of thewaste acid and precipitation and separation of the hydroxides of Ti, Al,Cr and V, as well as subsequent precipitation of magnetite with ammonia.However, the disadvantages of this process are that, on the one hand,large quantities of ammonia are consumed for the purpose of neutralisingthe free sulfuric acid and that the magnetite which is precipitated outof the solution containing a considerable amount of ammonium sulfatedisplays no pigment properties.

In the case of a modification, described in printed publication EP-A 638515, of the process in the form of an extraction of the magnesium fromthe solution containing ammonium sulfate, pure ammonium sulfate cansubsequently be obtained by crystallisation and can be used asfertiliser. Although the proportion of usable material is increased bythis means, the economy of the process is not satisfactory, by reason ofthe additional process steps and the inferior quality of the magnetiteobtained from the solution containing a considerable amount of ammoniumsulfate.

Another variant for reprocessing the waste acid consists, according toU.S. Pat. No. 4,137,292 and DE-A 24 56 320, in that gypsum and magnetiteare precipitated out simultaneously by neutralisation of the waste acidwith calcium compounds, whereby for utilisation of the two compounds amechanical separation, for example by means of a hydrocyclone or bymagnetic separation, has to be undertaken which, despite elaborateprocess steps, results neither in pure gypsum nor in a pure magnetitepigment. An optimisation of the process according to GB-A 1 421 773, tothe effect that ammonium salts or alkali-metal salts are presentsimultaneously in the course of precipitation of the gypsum with calciumcompounds, also does not avoid the aforementioned principaldisadvantages of this process.

The object was therefore to develop a process that makes it possible forthe waste acid accumulating during production of high-quality productsto be made available as extensively as possible for meaningfulutilisation.

Surprisingly it has been found that both a high-grade gypsum andhigh-grade iron-oxide pigments can be produced from the waste acidaccumulating in the course of the production of titanium dioxide inaccordance with the sulfate process if in a first step the waste acid iscaused to react with a calcium compound to form gypsum and the latter isseparated, optionally directly, from the remaining solution. In a secondstep, by increasing the pH value the elements Al, Ti, Cr, V and Fe(III)are at least partially precipitated out of the solution so obtained andare separated. In a third step an iron-oxide pigment is formed from theremaining solution containing iron(II) sulfate and is subsequentlyseparated from the rest of the solution.

SUMMARY OF THE INVENTION

The invention provides a process for the production of gypsum and alsoof an iron-oxide pigment from the waste acid accumulating in the courseof the production of titanium dioxide in accordance with the sulfateprocess, characterised in that in a first stage a partial neutralisationof the waste acid with a calcium compound is effected subject toprecipitation and optionally direct separation of gypsum, subsequentlythe remaining solution is neutralised further in a second stage subjectto precipitation of a deposit containing Ti, Al, Cr, V and optionallyFe(III), and the solution containing iron-sulfate that is obtained afterseparation of the solids is converted in a third stage into aniron-oxide pigment by adding alkaline compounds and also by addition ofan oxidising agent.

DETAILED DESCRIPTION OF THE INVENTION

Separation of the gypsum obtained in the first stage may be effecteddirectly after the precipitation, as a result of which a particularlyhigh quality is obtained; however, separation of the gypsum may also beeffected together with the solids precipitated in the second stage whichcontain Ti, Al, Cr, V and optionally Fe(III), as a result of which theprocess is simplified. Direct separation of the gypsum obtained in thefirst stage is preferred.

Partial neutralisation of the free sulfuric acid in the first stage ispreferably effected by addition of calcium carbonate, calciumhydrogencarbonate, calcium oxide or calcium hydroxide or alternativelyof other alkaline-reacting substances that contain one or more of thecited compounds, for example dolomite. However, partial neutralisationof the free sulfuric acid in the first stage may also be effected bymeans of a combination of individual substances cited above. Use ispreferably made of calcium carbonate, calcium oxide or calcium hydroxideas calcium source. The use of finely ground lime (calcium carbonate) isparticularly preferred, since the CO₂ arising in the process can also beutilised.

It is furthermore possible, simultaneously with the neutralisation ofthe waste acid, to raise the iron content with a view to increasing theamount of iron-oxide pigment through neutralisation of the free sulfuricacid being undertaken in part with substances that contain metallic ironor iron(II), for example with scrap iron, mill scale, turnings orcast-iron filings. Further neutralisation of the free sulfuric acid canthen be effected with calcium carbonate.

It is often expedient to dilute the waste acid prior to the reactionwith the calcium compound, in order to reduce the viscosity of thereaction mixture. For the purpose of dilution, use may be made either offresh water or alternatively of a process water that accumulates in thefurther course of the process. Dilution is preferably effected in a 1:1ratio (parts by weight of waste acid to parts by weight of water). It isadvantageous in this connection to slurry the Ca compound that is usedfor the purpose of neutralisation with a part of the dilution water andto neutralise the waste acid with this suspension.

Partial neutralisation of the waste acid in the first stage by additionof the calcium compound is preferably effected until a pH value from 1.0to 3.0 is attained. At higher pH values the degree of whiteness of thegypsum can be impaired by coprecipitation of coloured contaminants, forexample Fe(III) or other subgroup compounds. In particularly preferredmanner neutralisation is effected until a pH value from 1.4 to 2.0 isattained, since the gypsum precipitated in this way containsparticularly small amounts of coloured contaminants and particularlysmall amounts of TiO₂, so that it also meets the requirements forspecial applications, for gypsum plaster for example. The pure calciumsulfate obtained in this way is separated from the liquid phase and ispreferably washed with a view to removing coloured constituents of thesolution.

Alternatively, for the partial neutralisation of the free sulfuric acidin the first stage, instead of the calcium compound a correspondingbarium compound may also be chosen if barium sulfate is desired by wayof product.

Subsequently the solution is neutralised further in the second stage,preferably until a pH value from 3.0 to 5.0 is attained, in particularuntil a pH value from 3.5 to 4.8 is attained. In this process, inparticular in the course of the ensuing precipitation of magnetite,inconvenient titanium compounds and also, partially, Al, Cr, V andFe(III) compounds are precipitated out. The deposit obtained in this wayis separated from the liquid phase and may be either disposed of or usedas raw material for the extraction of Ti, Al, Cr or V.

Prior to the precipitation of the deposit containing Ti, Al, Cr and V,Fe(III) which is optionally present is preferably reduced to Fe(II) byaddition of a reducing agent, in particular metallic iron. As source ofthe metallic iron, substances resulting from industrial processes comeinto consideration in particular, such as turnings, cast-iron filings,stamped-metal waste or even mill scale. These substances are availablein large quantities at low cost. As a result, the amount of the depositresulting from the second precipitation stage to be disposed of orutilised further is reduced and the yield of high-grade iron-oxidepigment in the following stage is increased. This reduction of Fe(III)may be effected either prior to the precipitation of gypsum in the firststage or alternatively afterwards—ie, immediately prior to theprecipitation of the trivalent and tetravalent metallic ions. In thecourse of the reduction of the Fe(III), however, the reaction should becontrolled in such a way that as little as possible Ti³⁺ arises whichwould only be precipitated out incompletely in the second stage.Precipitation of the titanium in the second stage can optionally also beimproved by addition of seeds consisting of titanium oxide hydrate.

For this second neutralisation, by way of neutralising agent use may bemade of a compound that forms no sparingly soluble sulfates, so that theaccumulation of solids is minimised. Alternatively, however, by way ofneutralising agent use may also be made of a compound that forms readilyfilterable, sparingly soluble sulfates and therefore clearly improvesthe filterability of the entire quantity of solids accumulating.Finally, it is also possible for a combination of neutralising agents,one of which forms soluble sulfates and another of which forms sparinglysoluble sulfates, to find application with a view to selectiveoptimisation of this process step.

Suitable by way of neutralising agents are compounds from the groupcomprising gaseous NH₃, NH₃ dissolved in water, oxides, hydroxides,carbonates and hydrogen-carbonates of the alkali metals oralkaline-earth metals, as well as alkaline-reacting mixtures thatcontain at least one of the cited compounds. For reasons of economy theuse of alkaline-reacting ashes as neutralising agent is particularlyadvantageous, for example ashes resulting from the combustion of coal orfrom the incineration of refuse. Particularly preferred is thesimultaneous use of ammonia and alkaline-reacting ash or thesimultaneous use of caustic-soda solution and ash by way of neutralisingagent. In this connection use is preferably made of ammonia orcaustic-soda solution predominantly and of ash only in a smallproportion, in particular >90% alkali equivalents of ammonia orcaustic-soda solution and <10% alkali equivalents of ash. With thismanner of proceeding the small proportion of ash brings about a clearimprovement in the filterability of the metal hydroxides, whereas thequantity of solids accumulating is not increased significantly. Insteadof ash, use may also be made of an alkaline calcium compound, forexample CaO, but as a rule the use of ash is more attractiveeconomically.

The solution containing iron sulfate that is obtained after the secondneutralisation stage and separation of the solids is adjusted for theprecipitation of the iron-oxide pigment preferably to a concentration of150 to 250 g, in particular 180 to 190 g, FeSO₄ per liter. Thisconcentration can optionally be achieved by evaporation. The adjustedsolution containing iron sulfate is converted in known manner into aniron-oxide pigment, preferably a magnetite pigment, by addition ofalkaline compounds and addition of an oxidising agent, preferably byblowing in oxygen or gases containing oxygen, in particular air,(Ullmann's Encyclopaedia of Industrial Chemistry, 5^(th) Edn., Vol. A20, p 297 ff). By way of alkaline-reacting compounds use is preferablymade in this connection of those which form no sparingly solublesulfates, such as, for example, NH₃, NaOH, KOH, Na₂CO₃, K₂CO₃, MgO,MgCO₃ or Mg(OH)₂.

For the preferred production according to the invention of a blackiron-oxide pigment, preferably such a quantity of an alkaline-reactingprecipitating agent is added to the solution containing iron sulfatethat the ratio of iron(II) ions to precipitating agent amounts to 0.4 to0.65, in particular 0.5 to 0.58, equivalents. If, for example, NaOH isemployed by way of precipitating agent, then between 40 and 65 molesFeSO₄ can be employed for 100 moles NaOH. Where use is made of K₂CO₃ asprecipitating agent, between 80 and 130 moles FeSO₄ are employed for 100moles K₂CO₃. The calculated quantity of the alkaline component ispreferably added to the solution containing iron sulfate at atemperature between 60 and 95° C., in particular between 75 and 95° C.It is also possible to submit the alkaline component.

Oxidation is subsequently effected with an oxidising agent. By way ofoxidising agent use may be made, for example, of compounds from thegroup comprising oxygen, ozone, H₂O₂, sodium hypochlorite,sodium-hypochlorite solution, chlorates, perchlorates, nitrates andchlorine. Oxygen or a gas mixture containing oxygen, in particular air,is preferably introduced into the reaction mixture. The oxidation iscompleted as soon as the Fe(II) content of the suspension is less than 1mole-%.

Reprocessing of the pigment suspensions is effected by means of theknown steps of filtration, drying and grinding. A person skilled in theart will be able, by suitable variation of the production conditions, toproduce a broad palette of iron-oxide pigments of various particle sizesand consequently of various tones and stabilities.

By annealing the black iron-oxide pigment obtained it is possible toproduce high-grade brown iron-oxide pigments. In this connection thetone of the brown iron-oxide pigments can be fixed within certain limitsby varying the annealing conditions.

In this way both a high-grade gypsum and a high-grade iron-oxidepigment, each in pure form, are obtained from the waste acid, and theresulting waste is minimised.

Particularly advantageous in the case of the three-stage process withthe separate separation of the gypsum from the first stage is the factthat the gypsum obtained in accordance with the invention containsparticularly small amounts of colouring contaminants such as areconstituted by compounds of the elements Fe, Cr, V, for example.Furthermore, the gypsum contains particularly small amounts of Ticompounds, making it suitable also for the production of high-gradegypsum plasters or for the production of anhydrite. The iron-oxidepigment obtained in accordance with the invention is not contaminated bygypsum and exhibits particularly good optical pigment properties, sincethe parent solution containing iron sulfate which is used for thispurpose on the one hand contains particularly small amounts of titaniumsalts and, on the other hand, by reason of the precipitation of gypsumin the first stage does not have a very high concentration of neutralsalts, for example Na₂SO₄, K₂SO4 or (NH₄)₂SO₄. Furthermore, by reason ofthe preliminary precipitation of the metal hydroxides in the secondstage the iron-oxide pigment produced in accordance with the inventionis low in heavy-metal compounds, particularly in compounds of Cr and V.By virtue of the process according to the invention it is possible toprocess the waste acid that accumulates as waste with simpleprocess-engineering operations and with a comparatively low energydemand to yield high-grade products in the form of pure gypsum, ironoxide having pigment properties and pure CO₂ and to minimise the waste.The amount of waste from 7 to 8 t waste acid per t TiO₂ is preferablyreduced to only 0.2 to 0.7 t filter cake (from the second stage) per tof TiO₂ produced.

If the separation of the gypsum that is precipitated out in the firststage is effected jointly with the separation of the metal hydroxidesthat are precipitated out in the second stage and of the gypsum that isadditionally precipitated out in the second stage, no residues of anykind any longer accumulate that have to be disposed of or reprocessed(see Example 6).

The iron-oxide pigments obtained in accordance with the invention can beused for the dyeing of paints, lacquers, plastics, building materials,paper and other materials. The magnetite obtained in accordance with theinvention may furthermore also find application as magnetic pigment fortoner.

The tone of the iron-oxide pigment obtained is determined in accordancewith the following directions:

Measurement of the Mass Tone of Iron-oxide Pigments:

The pigment is dispersed with a muller (plate-type automatic muller) inan air-drying lacquer system. The lacquer system (lacquer) consists ofthe following components:

95.26%  ® ALKYDAL F 48 (binder, Bayer AG, medium-oily, air-drying alkydresin based on desiccative vegetable fatty acids in white-spirit/xylenemixture 38:7 with a non-volatile portion of about 55%, oil-con-tent/triglyceride in the non-volatile portion about 48%, phthalic an-hydride in the non-volatile portion about 26%) 0.78% 2-butanone oxime,55% in white spirit (anti-skinning agent) 1.30% ® Octa Soligen Calcium(wetting agent, calcium salt of branched C₆-C₁₉ fatty acids in a mixtureof hydrocarbons (contains 4% Ca), Borchers AG) 0.22 % ® Octa SoligenKobalt 6 (desiccant, cobalt(2+) salt of branched C₆-C₁₉ fatty acids in amixture of hydrocarbons (contains 6% Co), Borchers AG) 0.87% ® OctaSoligen Zirkonium 6 (desiccant, zirconium salt of branched C₆- C₁₉ fattyacids in a mixture of hydrocarbons (contains 6% Zr), Borchers AG) 1.57%Glycolic-n-butyl ester (= hydroxyethanoic butyl ester)(flow improver)

The components are intermixed with a high-speed stirrer to produce thefinished lacquer. Use is made of a plate-type automatic muller asdescribed in DIN EN ISO 8780-5 (April 1995). An ®ENGELSMANN JEL 25/53muller with an effective plate diameter of 24 cm is used. The speed ofrotation of the lower plate amounts to about 75 min⁻¹. As a result ofhanging a 2.5 kg loading weight on the loading frame the force betweenthe plates is adjusted to about 0.5 kN. 0.8 g pigment and 2.00 g lacquerare dispersed in a stage at 100 revs with a 2.5 kg loading weight inaccordance with the process described in DIN EN ISO 8780-5 (April 1995)Section 8.1. The muller is opened and the lacquer is swiftly collectedon the lower plate outside the midpoint. Then a further 2.00 g lacquerare added and the plates are closed together. After two stages at 50revs without loading weight the preparation is completed.

The pigmented lacquer is spread with a film spreader (gap height atleast 150 μm, at most 250 μm) on a non-absorbent cardboard. Thelacquered cardboard (coating) is then dried for at least 12 h at roomtemperature. Prior to the colour measurement the coating is dried forone hour at about 65° C. (±5° C.) and cooled.

Measurement of the Tint Tone of Iron-oxide Pigments:

The pigment and the brightening agent are dispersed with a muller(plate-type automatic muller) in an air-drying lacquer system. By way ofbrightening agent use is made of a commercially available ®BayertitanR-KB-2 titanium-dioxide pigment (Bayer AG). This pigment corresponds toType R 2 in ISO 591-1977. The lacquer system (lacquer) corresponds tothat for determining the mass tone (see above).

The components of the lacquer system are intermixed with a high-speedstirrer to produce the finished lacquer. The pigmented lacquer and thelacquer coating are produced in the manner described in connection withthe determination of the mass tone (see above), 0.1500 g pigment to betested, 0.7500 g Bayertitan R-KB-2 and 2.00 g lacquer being weighed in.

Measurement of the mass tone of gypsum on powder compact:

In order to produce the powder compact, 10 g of the gypsum sample werecompacted linearly from 0 to 120 bar within 30 seconds in a hydraulicpress and were maintained at 120 bar for 6 seconds. Colorimetriccharacterisation of the powder compact is effected in a manner analogousto the colorimetric characterisation of lacquer coatings.

Colour-measuring Instrument:

Use is made of a spectrophotometer (“colour-measuring instrument”)having an U1-bricht sphere with measurement geometry d/8 without glosstrap. This measurement geometry is described in ISO 7724/2-1984(E) point4.1.1, in DIN 5033 Part 7 (July 1983) point 3.2.4 and in DIN 53 236(January 1983) point 7.1.1. Use is made of a ®Dataflash 2000 measuringinstrument available from Datacolor International.

The colour-measuring instrument is calibrated against a white, ceramicworking standard as described in ISO 7724/2-1984 (E) point 8.3. Thereflection data of the working standard compared to an ideallydull-white body are stored in the colour-measuring instrument, so thatafter calibration with the white working standard all colourmeasurements are related to the ideally dull-white body. Calibration ofthe black point is carried out with a black hollow body available fromthe manufacturer of the colour-measuring instrument.

Colorimetry

Any gloss trap which may be present is disconnected. The temperature ofthe colour-measuring instrument and of the test piece amounts to about25° C.±5° C.

The lacquer coating is placed onto the colour-measuring instrument insuch a way that the measuring hole is covered by a central point of thelayer of lacquer. The entire coating has to lie flat. The measuring holehas to be totally covered by the layer of lacquer. The measurement isthen carried out.

Calculation of the CIE Coordinates:

From the measured reflection spectrum the CIE coordinates L*, a* and b*of 1976 are calculated in accordance with the calculating instructionsgiven in ASTM E 308-1985, point 7. Use is made of the weightingfunctions of the standard illuminant C and of the 2° standardcalorimetric observer of 1931 given in ASTM E 308-1985, Table 5.6. Thewavelength range is between 400 nm and 700 nm. The wavelength intervalamounts to 20 nm. No gloss is subtracted in the calculation. Thereflectance values obtained are converted in accordance with DIN 5033,Part 3 (July 1992) into the values pertaining to the CIELAB colour datasystem.

The relative colour intensity is calculated by analogy with the relativescattering power according to DIN 53 165 (point 3.4) with BayertitanR-KB-2 as brightening agent and with a suitable Bayferrox referencepigment (instead of carbon black); by way of ρ∝ use is made of thetristimulus value Y/100.

The invention is described below on the basis of Examples, without anylimitation being constituted thereby. The parts and percentages quotedin the Examples relate to weight, unless otherwise stated.

EXAMPLES Example 1

100 kg waste acid having a content of free sulfuric acid of 24.45% andan Fe content of 2.95% are diluted with 50 kg water and heated up to 80°C. Into this solution there is pumped a suspension consisting of 25.94kg ground calcium carbonate (53.9 % CaO; 43.1% CO₂) and 50 kg waterwithin 1.5 h subject to stirring. After addition is complete, stirringis effected for a further 2 h. The pH value of the reaction mixtureamounts after this to 1.5. The CO₂ arising can be collected, optionallypurified and compressed. The gypsum precipitated out is filtered off viasuction filters and washed with 74 kg of a 0.167-% sulfuric acid(pH=1.5). 88.83 kg filtrate, 80.7 kg wash filtrate and 101.5 kg filtercake with a solids content of 50.42% (60° C. until constancy of weight)are obtained. After drying of the filter cake in a circulating-airdrying cabinet at 60° C. 51.18 kg calcium sulfate dihydrate are obtainedhaving the following properties:

Residual moisture: 0.03% (24 h at 40° C.): Ca: 20.0% SO₄: 54.9% CO₃:0.07% Fe: 1.25% Ti: 0.78% Mg: 0.3% Mn: 0.03% Cr: 0.021% V: 0.054% Al:0.2% Na: 0.071% DIN pH: 2.76 BET: 21.1 m²/g Mass tone on powder compact:L* = 87.5 ΔL* = −6.2 (compared to enamel tile BAM SIE 0259/05 b) ΔL* =10.0 (compared to Bayertitan R-KB-2)

To 67.4 kg of the filtrate obtained in the course of the separation ofthe gypsum in the first stage (35.3 kg filtrate and 32.1 kg washfiltrate; pH value 1.78) a suspension consisting of 0.464 kg CaO and4.176 kg water is pumped in at 75° C. during a period of 2 h. The pHvalue of the reaction mixture amounts after this to 4.5. The depositprecipitated out is filtered off via suction filters and washed with 1.5kg water. 56.3 kg filtrate, 2.08 kg wash filtrate and 3.66 kg filtercake with a solids content of 48.3 % (60° C. until constancy of weight)are obtained. After drying of the filter cake in a circulating-airdrying cabinet at 60° C. 1.768 kg solids are obtained having thefollowing properties:

Residual moisture: 0.21% (24 h at 40° C.): Ca: 16.8% SO₄: 45.0% CO₃:0.05% Fe: 1.8% Ti: 0.39% Mg: 0.13% Mn: 0.014% Cr: 0.34% V: 0.50% Al:5.1% Na: 0.032% DIN pH: 4.41 BET: 26.1 m²/g Mass tone on powder compact:L* = 79.4 ΔL* = −14.3 (compared to enamel tile BAM SIE 0259/05 b) ΔL* =−18.1 (compared to Bayertitan R-KB-2)

58.4 kg of the solution containing iron sulfate that is obtained afterseparation of the filter cake in the second stage (56.3 kg filtrate and2.08 kg wash filtrate; set to pH<2 with sulfuric acid during interimstorage in order to prevent oxidation; FeSO₄ concentration 42 g/l) aresubmitted in a stirrer vessel with stirrer and gassing device and heatedto 85° C. After this, 5.72 kg of a 24-% caustic-soda solution (300 gNaOH/l) are added, in order to adjust the pH value of the reactionmixture to 7.0. Subsequently gassing is effected with 1 m³/h air, the pHvalue being maintained constant at 7.0 by further addition of NaOH,until a jump in potential occurs in the solution from about -700 mV toabout -200 mV (after about 4.5 h). The magnetite precipitated out isfiltered off via suction filters and washed with 5.0 kg water. 50.35 kgfiltrate, 4.1 kg wash filtrate and 1.76 kg filter cake with a solidscontent of 67.4% (60° C. until constancy of weight) are obtained. Afterdrying of the filter cake in a circulating-air drying cabinet at 60° C.and deagglomeration with a cross-beater mill, 1.186 kg magnetite areobtained having the following properties:

Ca: 0.045% SO₄: 1.24% Fe: 68.45% Ti: 0.011% Mg: 0.22% Mn: 1.0% Cr:0.005% V: 0.004% Al: 0.043% Na: 0.11% BET: 7.5 m²/g Mass tone: L* = 12.4ΔL* = −0.4 (compared to Bayferrox 330) a* = 0.8 Δa* = 0.1 (compared toBayferrox 330) b* = −0.2 Δb* = −0.9 (compared to Bayferrox 330) Tinttone: L* = 52.9 ΔL* = 3.1 (compared to Bayferrox 330) a* = 0.5 Δa* = 0.1(compared to Bayferrox 330) b* = −4.2 Δb* = 1.0 (compared to Bayferrox330)

The relative colour intensity compared to Bayferrox 330 amounts to 131%.

The Magnetite Obtained is also Suitable as a Magnetic Pigment for Toner:

Saturation=1,027 Gauss.cm³/g

Remanence=133 Gauss.cm³/g

Coercivity=61.9 Oerstedt

Example 2

100 kg waste acid having a content of free sulfuric acid of 24.45% andan Fe content of 2.95% are diluted with 50 kg water and heated up to 80°C. Into this solution there is pumped a suspension consisting of 25.94kg ground calcium carbonate (53.9 % CaO; 43.1% CO₂) and 50 kg waterwithin 1.5 h subject to stirring. After addition is complete, stirringis effected for a further 2 h. The pH value of the reaction mixtureamounts after this to 1.5. The CO₂ arising can be collected, optionallypurified and compressed. The gypsum precipitated out is filtered off viasuction filters and washed with 74 kg of a 0.167-% sulfuric acid(pH=1.5). 88.83 kg filtrate, 80.7 kg wash filtrate and 101.5 kg filtercake with a solids content of 50.42% (60° C. until constancy of weight)are obtained. After drying of the filter cake in a circulating-airdrying cabinet at 60° C. 51.18 kg calcium sulfate dihydrate are obtainedhaving the following properties:

Residual moisture: 0.03% (24 h at 40° C.): Ca: 20.0% SO₄: 54.9% CO₃:0.07% Fe: 1.25% Ti: 0.78% Mg: 0.3% Mn: 0.03% Cr: 0.02% V: 0.054% Al:0.2% Na: 0.071% DIN pH: 2.76 BET: 21.1 m²/g Mass tone on powder compact:L* = 87.5 ΔL* = −6.2 (compared to enamel tile BAM SIE 0259/05 b) ΔL* =−10.0 (compared to Bayertitan R-KB-2)

Into 67.4 kg of the filtrate obtained in the course of the separation ofthe gypsum in the first stage (35.3 kg filtrate and 32.1 kg washfiltrate; pH value 1.78) 0.214 kg NH₃ gas is introduced at 75° C. duringa period of 1 h. The pH value of the reaction mixture amounts after thisto 4.5. The deposit precipitated out is filtered off via suction filtersand washed with 1.5 kg water. 61.7 kg filtrate, 1.9 kg wash filtrate and2.038 kg filter cake with a solids content of 30.9% (60° C. untilconstancy of weight) are obtained. After drying of the filter cake in acirculating-air drying cabinet at 60° C. 0.63 kg solids are obtainedhaving the following properties:

Ca: 0.3% Fe: 3.5% Ti: 2.1% Mg: 0.34% Mn: 0.027% Cr: 1.5% V: 2.5% Al:17.7% NH₄: 0.41% BET: 45.3 m^(/)g

63.6 kg of the solution containing iron sulfate that is obtained afterseparation of the filter cake in the second stage (61.7 kg filtrate and1.9 kg wash filtrate; set to pH<2 with sulfuric acid during interimstorage in order to prevent oxidation; FeSO₄ concentration 45 g/l) aresubmitted in a stirrer vessel with gassing device and heated to 85° C.subject to N₂ screening. After this, 2.003 kg NH₃ are introduced duringa period of about 3.3 h, in order to adjust the pH value of the reactionmixture to 7.0. Subsequently gassing is effected with 1 m³/h air, the pHvalue being maintained constant at 7.0 by further addition of NH₃, untila jump in potential occurs in the solution from about -700 mV to about-200 mV (after about 5 h). The magnetite precipitated out is filteredoff via suction filters and washed with 5.0 kg water. 52.06 kg filtrate,4.76 kg wash filtrate and 2.39 kg filter cake with a solids content of54.76% (60° C. until constancy of weight) are obtained. After drying ofthe filter cake in a circulating-air drying cabinet at 60° C. anddeagglomeration with a cross-beater mill, 1.31 kg magnetite are obtainedhaving the following properties:

Ca: 0.020% Fe: 64.6% Ti: 0.011% Mg: 1.0% Mn: 0.79% Cr: 0.034% V: 0.10%Al: 0.71% NH₄: <0.03% BET: 12.8 m²/g Mass tone: L* = 13.2 ΔL* = 0.4(compared to Bayferrox 330) a* = 0.9 Δa* = 0.0 (compared to Bayferrox330) b* = 0.8 Δb* = 0.1 (compared to Bayferrox 330) Tint tone: L* = 56.5ΔL* = 0.2 (compared to Bayferrox 330) a* = 0.7 Δa* = 0.1 (compared toBayferrox 330) b* = −2.7 Δb* = 0.5 (compared to Bayferrox 330)

The relative colour intensity compared to Bayferrox 330 amounts to 98%.

The Magnetite Obtained is also Suitable as a Magnetic Pigment for Toner:

Saturation=1,061 Gauss.cm³/g

Remanence=221 Gauss.cm³/g

Coercivity=68.5 Oerstedt

Example 3

20 g of the magnetite obtained in accordance with Example 1 arebiscuit-fired to yield a brown iron-oxide pigment by the magnetitelocated in a ceramic dish being heated in a chamber furnace in a currentof air amounting to 600l/h with a rate of heating of 4° C./min and beingtaken out of the furnace at 600° C.

After a grinding stage lasting 30 s in a pin-disk disintegrator a browniron-oxide pigment is obtained having the following properties:

Mass tone: L = 32.5 a* = 16.2 b* = 9.9 ΔL* = 0.5 (compared to Bayferrox180 M) Δa* = −2.3 (compared to Bayferrox 180 M) Δb* = 1.5 (compared toBayferrox 180 M) Tint tone: L* = −59.2 a* = 13.0 b* = 2.6 ΔL* = −7.3(compared to Bayferrox 180 M) Δa* = 1.0 (compared to Bayferrox 180 M)Δb* = 2.2 (compared to Bayferrox 180 M)

The relative colour intensity compared to Bayferrox 180 M amounts to196%.

Example 4:

10 kg waste acid (composition as in Example 1) are diluted with 0.5 kgwater and heated up to 80° C. Into this solution there is pumped asuspension consisting of ground calcium carbonate and 0.5 k water withina period of 1.5 h subject to stirring until a pH value of 2.5 isattained. After addition is complete, stirring is effected for a further2 h. The CO₂ arising can be collected, optionally purified andcompressed. The gypsum precipitated out is filtered off via suctionfilters and washed with dilute sulfuric acid (pH=2.5).

After filtration and drying of the filter cake in a circulating-airdrying cabinet at 60° C. a calcium sulfate dihydrate is obtained whichin comparison with the corresponding product from Example 1 has asomewhat higher titanium content but a comparable degree of whiteness.

Apart from use as gypsum plaster, this material is just as suitable formost other fields of application as the corresponding product fromExample 1.

Further processing of the filtrate to yield a black iron-oxide pigmentis effected in a manner analogous to Example 1.

Example 5

A gypsum that has been precipitated out and filtered off via suctionfilters in a manner analogous to Example 1 is washed with 102 kg(instead of 74 kg as in Example 1) of a dilute sulfuric acid (pH=1.94).After drying of the filter cake in a circulating-air drying cabinet at60° C. calcium sulfate dihydrate is obtained having the followingproperties:

Ca: 22.5% SO₄: 55.4% CO₃: 0.28% Fe: 0.10% Ti: 0.45% Mg: 0.018% Mn:0.003% Cr: 0.019% V: 0.042% Al: 0.17%

As a result of the more intensive washing a particularly pure calciumsulfate dihydrate is obtained.

Further processing of the filtrate to yield a black iron-oxide pigmentis effected in a manner analogous to Example 1.

Example 6

40 kg waste acid having a content of free sulfuric acid of 24.45% and anFe content of 2.95% are diluted with 20 kg water and heated up to 80° C.Into this solution there is pumped a suspension consisting of 10.3 kgground calcium carbonate (53.9 % CaO; 43.1% CO₂) and 20 kg water withina period of 1.5 h subject to stirring. The pH value of the reactionmixture amounts after this to 1.5. The CO₂ arising can be collected,optionally purified and compressed. Subsequently a suspension consistingof 0.464 kg CaO and 4.176 kg water is pumped in during a period of 2 h.The pH value of the reaction mixture amounts after this to 4.5. Thedeposit precipitated out is filtered off via suction filters and washedwith about 75 kg water. After drying of the filter cake in acirculating-air drying cabinet at 60° C. about 22 kg solids are obtainedhaving the following properties:

Ca: 19.7% SO₄: 54.1% Fe: 1.29% Ti: 0.75% Mg: 0.29% Mn: 0.020% Cr: 0.047%V: 0.098% Al: 0.59% Na: 0.068% Mass tone on powder compact: L* = 87.3ΔL* = −6.5 (compared to enamel tile BAM SIE 0259/05 b) ΔL* = −10.3(compared to Bayertitan R-KB-2)

for comparison:

Mass tone on powder compact of gypsum resulting from flue-gasdesulfurisation (REA gypsum):

L*=74.8

The solid obtained, predominantly containing gypsum, has a comparablecomposition to and only a slightly lower degree of whiteness than thegypsum obtained in the first precipitation stage of Example 1 and isequally suitable for most applications. A comparison with gypsumresulting from a flue-gas desulfurisation plant shows that the degree ofwhiteness of the gypsum obtained in accordance with the invention liesclearly above that of REA gypsum.

Further processing of the filtrate to yield a black iron-oxide pigmentis effected in a manner analogous to Example 1.

Example 7

Precipitation and separation of the gypsum in the first stage areeffected in a manner analogous to Example 1.

Subsequently 1.685 kg of the solution that is obtained after separationof the gypsum (filtrate and wash filtrate) are adjusted to a pH value of4.5 by addition of 9.3 g power-station ash (with a content of free CaOof 0.95%) and then 5.3 g NH₃, and the deposit obtained in the process isseparated by filtration. The filter cake obtained is washed with 100 gwater. 83 g filter cake are obtained with a solids content of 20.5%.

Alternatively, neutralisation may be undertaken by means of NH₃ only(without the use of power-station ash) until a pH value of 4.5 isattained.

Neutr'n with ash Neutr'n w/o ash Amount of solution 1.685 kg 1.685 kgAddition of power-station ash 9.3 g — Addition of NH₃ 5.3 g 5.4 g Filtercake (moist) 109 g 83 g Solids content 24.7% 20.5% Filter cake (dry)26.9% 17.0% Filtering-time 190 s 174 s Washing-time 40 s 90 s

It is evident that where use is made of power-station ash by way offilter aid the washing-time is clearly shorter, despite the largeramount of filter cake.

Further conversion into a magnetite pigment is effected in a manneranalogous to Example 1.

Example 8 (Comparative Example)

20 kg waste acid are introduced into a stirrer vessel at 70 to 80° C.simultaneously with 5.00 kg NH₃ and are neutralised at a constant pHvalue of 5.0. In the process 0.38 kg of a deposit containing metalhydroxide are obtained, said deposit being separated by filtration. Theremaining solution (54 kg) contained about 36% (NH₄)₂SO₄, 4.2% FeSO₄,0.08% MnSO₄, 3.3% MgSO₄ and about 56% water.

This solution is oxidised in a second stage with about 2 m³ air and ismaintained during the oxidation at a pH value of 7.0 by further additionof 0.51 kg NH₃. In this process 1.14 kg solids (magnetite) and also 53.5kg ammonium-sulfate solution with about 40.0% (NH₄)₂SO₄ are obtainedafter filtration.

Characterisation according to the method described in the text yieldsthe following:

Tint tone: L* = 64.5 a* = 0.0 b* = −0.4 Tint tone (relative to Bayferrox306): ΔL* = 1.6 Δa* = −0.1 Δb* = 0.4

Since the magnetite obtained from the highly concentratedammonium-sulfate solution exhibits a tint tone that is too high or acolour intensity that is too low, it is not suitable as a pigment.

What is claimed is:
 1. A process for the production of gypsum and alsoan iron oxide pigment from the waste acid accumulating during theproduction of titanium dioxide by the sulfate process, comprising a) ina first stage partially neutralizing the waste acid with a calciumcompound to precipitate gypsum and obtain a remaining solution, b)subsequently neutralizing the remaining solution in a second stage witha neutralizing agent to precipitate a solid containing Ti, Al, Cr, V andoptionally Fe(III) and c) producing in a third stage, from the solutioncontaining iron sulfate that is obtained after separation of the solid,an iron oxide pigment by addition of an alkaline compound and anoxidizing agent.
 2. The process of claim 1 comprising separating jointlyand utilizing the gypsum obtained in the first stage and the solidcontaining Ti, Al, Cr, V and optionally Fe(III) obtained in the secondstage.
 3. The process of claim 1 comprising separately separating thegypsum obtained in the first stage and separately separating the solidobtained in the second stage.
 4. The process of claim 1 comprisingpartially neutralizing the waste acid in the first stage by addition ofthe calcium compound until a pH of 1.0 to 3.0 is reached.
 5. The processof claim 1 wherein the calcium compound in the first stage comprisescalcium carbonate, calcium hydrogen carbonate, calcium oxide or calciumhydroxide.
 6. The process of claim 1 wherein the calcium compound in thefirst stage comprises an alkaline-reacting compound containing calciumcarbonate, calcium hydrogencarbonate, calcium oxide or calciumhydroxide.
 7. The process of claim 1 comprising adding metallic iron ora substance containing metallic iron either prior to or after theprecipitation of gypsum in the first stage.
 8. The process of claim 1comprising further neutralizing the solution in the second stage until apH value from 3.0 to 5.0 is reached and separating the solid obtainedfrom the liquid phase.
 9. The process of claim 1 comprising using in thesecond stage a compound as neutralizing agent that forms no sparinglysoluble sulfates.
 10. The process of claim 1 comprising using in thesecond stage two or more different compounds as neutralizing agents, atleast one of which forms sparingly soluble sulfates and at least one ofwhich forms soluble sulfates.
 11. The process of claim 1 wherein theneutralizing agent in the second stage comprises an alkali oxide, analkali hydroxide, an alkali carbonate, an alkali hydrogen carbonate, analkaline earth oxide, an alkaline earth hydroxide, an alkaline earthcarbonate or an alkaline earth hydrogen carbonate.
 12. The process ofclaim 1 wherein in the second stage the neutralizing agent comprisesgaseous NH₃ or NH₃ dissolved in water.
 13. The process of claim 1wherein in the second stage the neutralizing agent comprises powerstation ash, refuse incineration ash or other alkaline-reacting ash. 14.The process of claim 1 comprising adjusting the iron sulfateconcentration between 150 and 250 g FeSO₄ per liter following the secondneutralization stage after the separation of the solids, optionally byevaporative concentration.
 15. The process of claim 1 wherein thesolution containing iron sulfate is converted into a black iron oxidepigment by addition of an alkaline compound that does not form sparinglysoluble sulfates and by addition of an oxidizing agent.
 16. The processof claim 1 comprising converting the solution containing iron sulfatethat is obtained after the second neutralization stage into a black ironoxide pigment by adding an alkaline compound comprising a memberselected from the group consisting of gaseous NH₃, NH₃ dissolved inwater, NaOH, KOH, MgO, MgCO₃ or Mg(OH)₂ and by blowing in oxygen orgases containing oxygen.
 17. The process of claim 1 comprisingbiscuit-firing the black iron oxide pigment to yield a brown iron oxidepigment.